1. Field of the Invention
The present invention relates to an objective lens element for use in an optical head device that performs at least one of recording, reproducing, and erasing of information on an optical information storage medium.
2. Description of the Background Art
As media that record a large amount of information with a high density, there are optical information storage media such as optical discs. Optical discs record information as pit-shaped patterns thereon, and are widely used for the purposes of recording digital audio files, video files, document files, and data files. Examples of functions required for performing recording, reproducing, and erasing of information on an optical disc with high reliability by using a light beam converged on a micro spot are a converging function to form a diffraction-limited micro spot, focus control (focus servo) of an optical system, tracking control, and pit signal (information signal) detection.
In recent years, due to advancement of optical system design technology and shortening of the wavelengths of semiconductor lasers which are light sources, development has progressed concerning optical discs that have a higher-density storage capacity further than ever. One approach to density increase is to increase the optical disc-side numerical aperture (NA) of a light-converging optical system which converges a light beam to form a micro spot on the optical disc. However, when the NA of the light-converging optical system is increased, an amount of a generated aberration increases with respect to a certain amount of tilt of the optical axis. In order to prevent this problem, it is necessary to decrease the thickness of a layer (hereinafter, referred to as “base material thickness”) provided on a recording surface of the optical disc. In the present specification, the “base material thickness” means a thickness from a light beam incident surface to an information recording surface of an optical disc.
For compact discs (CD) which are first generation optical discs, infrared light (a wavelength λ3: 780 to 820 nm) and an objective lens having an NA of 0.45 are used. The base material thickness of CD is 1.2 mm. For DVD which is second generation, red light (a wavelength λ2: 630 to 680 nm) and an objective lens having an NA of 0.6 are used. The base material thickness of DVD is 0.6 mm. For third generation optical discs, blue light (a wavelength λ1: 390 to 415 nm) and an objective lens having an NA of 0.85 are used. The base material thickness of third generation optical discs is 0.1 mm. As described above, as the recording density increases, the base material thickness of the optical disc decreases.
In view of economical efficiency and space occupied by an apparatus, an optical information recording/reproducing apparatus is desired which can perform recording and reproducing on optical discs having different base material thicknesses and recording densities. For this, a light-converging optical system which can converge a light beam to a diffraction limit on a recording surface of each of optical discs having different base material thicknesses, and an optical head device including this light-converging optical system, are necessary. In addition, when recording and reproducing are performed on an optical disc having a large base material thickness, it is necessary to converge a light beam on a recording surface located deeper than a beam incident surface of the optical disc, and thus the focal length has to be increased.
Prior art documents disclose configurations intended for compatible reproducing and compatible recording on an optical disc having a base material thickness of 0.6 mm and compatible with the wavelength λ2 (red light) and on an optical disc having a base material thickness of 0.1 mm and compatible with the wavelength λ1 (blue light).
A first prior art example is a configuration in which a wavelength-selective phase plate is combined with an objective lens. This is disclosed in Japanese Laid-Open Patent Publication No. 10-334504 and the Proceedings of ISOM2001 (Session We-C-05), P30. The configuration disclosed in the Proceedings of ISOM2001 (Session We-C-05), P30 will be described with reference to FIGS. 21 and 22. FIG. 21 illustrates a schematic configuration of an optical head device. Parallel light emitted from a blue optical system 51 including a blue light source of a wavelength λ1 (405 nm) passes through a beam splitter 161 and a wavelength selection phase plate 205 and is converged by an objective lens 50 on an information recording surface of an optical disc 10 (third generation optical disc) having a base material thickness of 0.1 mm. The light reflected by the optical disc 10 travels along the reverse path and is detected with a detector of the blue optical system 51. Diverging light emitted from a red optical system 52 including a red light source of a wavelength λ2 (650 nm) is reflected by the beam splitter 161, passes through the wavelength selection phase plate 205, and is converged by the objective lens 50 on an information recording surface of an optical disc 10 (second generation optical disc: DVD) having a base material thickness of 0.6 mm. The light reflected by the optical disc 10 travels along the reverse path and is detected with a detector of the red optical system 52.
The objective lens 50 is designed such that when the parallel light of the wavelength λ1 is incident thereon, a convergence spot is formed at a position where the light has passed through a protective layer having a base material thickness of 0.1 mm. When recording and reproducing are performed on DVD, a spherical aberration occurs due to a difference in base material thickness. In order to compensate the spherical aberration, the light beam emitted from the red optical system 52 is adjusted to be diverging light, and the wavelength selection phase plate 205 is used. When light incident on the objective lens is adjusted to be diverging light, a new spherical aberration occurs. Thus, the spherical aberration occurring due to the difference in base material thickness can be cancelled by this new spherical aberration. Further, the wavefront is corrected also by the wavelength selection phase plate 205.
FIGS. 22A and 22B are a plan view and a cross-sectional view of the wavelength selection phase plate 205. The phase plate 205 has phase steps 205a of heights h and 3h. Here, the refractive index with respect to the wavelength λ1 is n1, and h=λ1/(n1−1). When the light of the wavelength λ1 is used, an optical path difference caused by the phase step of the height h is the used wavelength λ1 and corresponds to a phase difference of 2π. Thus, the optical path difference is the same as a phase difference of 0. Therefore, the phase steps 205a do not influence the phase distribution of the light of the wavelength λ1 and hence do not influence recording and reproducing on the optical disc 10. Meanwhile, when the light of the wavelength λ2 is used, designing is performed such that an optical path difference (h×(n2−1) which is provided by the step to the light of the wavelength λ2 has a value other than an integral multiple of the wavelength (e.g., 0.6 times). By utilizing the phase difference caused by the optical path difference, the aberration compensation described above is performed.
As a second prior art example, a configuration in which a refraction type objective lens and a diffraction element are combined is disclosed. In Japanese Laid-Open Patent Publication No. 2004-071134, in an optical head device which performs recording or reproducing on a high-density optical disc by using an objective lens having a high NA, a sawtooth-like diffraction element is used in order to be able to also perform recording or reproducing on conventional optical discs such as DVD. The sawtooth height is set such that when blue light is used, the length of the optical path becomes 2λ, and 2nd order diffracted light is used. The sawtooth-like diffraction element emits 1st order diffracted light when red light is incident thereon. The braze direction is as in a convex lens type, and chromatic aberration compensation of the refractive lens is performed. The diffraction order when red light is used is lower than the diffraction order when blue light is used. Thus, the sawtooth-like diffraction element serves as a concave lens for red light, thereby providing an effect that the working distance can be increased.
Further, Japanese Laid-Open Patent Publication No. 2004-071134 also discloses a diffraction element shown in FIG. 23A. The diffraction element shown in FIG. 23A has a stair-like cross-sectional shape and is composed of consecutive unit steps whose number is an integer number. Each unit step provides an optical path difference of about 1.25 wavelengths to a light beam of the wavelength λ1. Specifically, the wavelength λ1 is 390 to 415 nm, and a stair shape is provided in which one cycle consists of steps which are 0 times, 1 times, 2 times, and 3 times that of the unit step in height from the outer side of the diffraction element toward the optical axis. With respect to blue light, as shown in FIG. 23B, the phase changes in the same direction as that of the stair shape and a convex lens effect is exerted. With respect to red light, as shown in FIG. 23C, the phase changes in the direction opposite to that of the stair shape and a concave lens effect is exerted. Thus, when blue light is used, a chromatic aberration compensation effect of the refractive lens is obtained. In addition, when red light is used, an effect that the working distance (the interval between the objective lens surface and the surface of an optical disc) can be increased is obtained due to the concave lens effect.
As a third conventional art example, a configuration in which a relay lens is inserted between an infrared light source and an objective lens, thereby also realizing compatibility with a first generation optical disc having a base material thickness of 1.2 mm, is disclosed in Japanese Laid-Open Patent Publication No. 2004-281034.
Japanese Laid-Open Patent Publication Nos. 10-334504 and 2004-071134 merely disclose the method for compatibility with the above second generation optical discs and the above third generation optical discs. In addition, Japanese Laid-Open Patent Publication No. 2004-281034 discloses the method for compatibility with the above first generation optical discs, but requires a relay lens.
Further, it is desired that an element that realizes compatibility is integrally formed on the objective lens surface, in view of cost reduction by decrease in number of parts. However, in the conventional art described above, only the exemplary configuration, in which the phase plate or the diffraction element is provided independently of the refraction type objective lens, is disclosed, and there is no description about integrally forming an element, which realizes compatibility, on the objective lens surface.
Moreover, in order to produce objective lenses at low cost and in large quantities, the material of the objective lenses is preferably resin rather than glass. In general, the material cost of resin is low, and it is also possible to mold resin at a lower temperature than to mold glass. Thus, the mold can be used long and the molding time can be shortened. Therefore, by molding resin to produce objective lenses, the manufacturing cost can be reduced. However, the refractive index of a high-NA objective lens made of resin changes due to temperature change. The refractive index change causes the refractive power of the lens surface to shift from a designed value, whereby a spherical aberration occurs. A lower-order aberration greatly deteriorates the quality of an information reproduction signal, and thus a 3rd order spherical aberration is problematic.